As a constant velocity universal joint built into a drive shaft, a propeller shaft, and the like for transmitting a rotational force at a constant velocity from an engine of an automobile to a wheel, for example, there are two types that are a fixed type constant velocity universal joint and a plunging type constant velocity universal joint. Those constant velocity universal joints each have a structure capable of coupling the two shafts on the driving side and the driven side to each other, and transmitting rotational torque at a constant velocity even when each of the two shafts forms an operating angle.
The drive shaft for transmitting power from the engine of the automobile to a driving wheel needs to adapt to angular displacement and axial displacement due to changes in relative positional relationship between a differential and the wheels, and hence the drive shaft generally has a structure in which the plunging type constant velocity universal joint is mounted to the differential side (inboard side), the fixed type constant velocity universal joint is mounted to the driving wheel side (outboard side), and both the constant velocity universal joints are coupled to each other with the shaft. Typical examples of the fixed type constant velocity universal joint include a Rzeppa type constant velocity universal joint, and typical examples of the plunging type constant velocity universal joint include a double offset type constant velocity universal joint and a tripod type constant velocity universal joint.
The above-mentioned constant velocity universal joints need to be excellent in strength and durability, and hence solid metal materials (ingot materials) are generally used for components of those constant velocity universal joints. Thus, the components are finished into complete products by, for example, the steps of performing a forging process and the like so as to obtain a preform, performing a turning process on an outer surface, an inner surface, and the like, performing heat treatment such as quenching, and performing a grinding process on parts required to have high accuracy. In this way, the amounts of processing and material loss are large, and hence reduction in manufacturing cost is limited.
Meanwhile, the constant velocity universal joint described in Patent Document 1 includes a cage made of a sintered metal. However, measures to secure a requisite strength and a requisite durability of the constant-velocity universal joint, and measures in view of manufacture, such as that for mass-production of the constant velocity universal joint, need to be further studied.
Further, in view of use environment, for an automobile left in an extremely cold region over a long time period, a temperature of a fixed type constant velocity universal joint used to drive a front wheel thereof is as low as an outside temperature (for example, −20° C. or less), and a temperature of grease sealed therein is low as well. In such a case, when torque is input to the joint at a high operating angle, immediately after the input of the torque, stick-slip due to a temporary lack of lubricity may occur at portions in an inside of the joint, such as portions between track grooves and balls, between an inner spherical surface of an outer joint member and an outer spherical surface of the cage, and between an outer spherical surface of an inner joint member and an inner spherical surface of the cage. Such stick-slip may be unpleasant noise to driver's ears (this noise is referred to as “noise at low temperature”). The noise at low temperature is immediately deadened as a result of an increase in joint internal temperature due to heat generation along with rotation of the joint. Thus, the noise at low temperature is a problem only at the time of a travel start, in other words, not a problem except immediately after the travel start. However, it has been desired to take some countermeasure therefor.